The present invention relates to radio-frequency antenna combining systems and, more particularly, to a directional sector antenna combining system that provides an omnidirectional receiving pattern with substantially instantaneous acquisition of received radio frequency signals.
In order to provide a greater receiving coverage area for a radio station, a plurality of receiving antennas may be distributed throughout the coverage area, or a plurality of directional high-gain antennas arranged in an antenna system may be co-located near the center of the coverage area. In both of the foregoing instances, the received signals from the antennas may be combined at a central station to provide a composite signal or may be sampled in order to select the strongest of the received signals. For example, four spatially disposed omnidirectional antennas for receiving satellite signals have been combined in a shipboard receiving system. Another system, described in U.S. Pat. No. 4,128,740, provides at cellular antenna sites a plurality of sector antennas, each antenna accommodating RF signals for different radio channels.
However, when directional antennas have been utilized, prior art receiving systems have utilized antenna signal scanning methods instead of combining. An illustrative example is the antenna scanning system described in a paper by C. Chuag and R. Hollister, entitled "A New Multiple-Beam Antenna For 360 Degree Coverage", published in the 1978 International Symposium Digest on Antennas and Propagation by the IEEE at pp. 118-121. In an antenna scanning system, the central station chooses the antenna receiving the strongest signal by comparing the signal levels received on each directional antenna in succession. The sampling and comparing techniques required to select the directional antenna with the strongest signal require as much as 50 milliseconds per directional antenna. Thus, if six directional antennas are to be scanned, a total time of 300 milliseconds is required. A signal acquisition time of 300 milliseconds is not tolerable in some radio systems. For example, systems transmitting information in digital bit streams may loose some of the digital information because of a long signal acquisition time.
The signal acquisition time of a receiving system will be minimal when one omnidirectional receiving antenna is utilized. An omnidirectional antenna will exhibit considerably less antenna gain than a directional gain antenna having similar physical dimensions, however, resulting in a considerable reduction in the size of the receiving coverage area. Thus, none of the foregoing prior art receiving systems are capable of providing both a fast acquisition time to the received signal and a large omnidirectional receiving coverage area.
Accordingly, it is an object of the present invention to provide an improved instantaneously acquiring directional receiving antenna combining system having a large substantially omnidirectional receiving coverage area.
It is another object of the present invention to provide an improved instantaneously acquiring directional receiving antenna combining system that provides a space diversity improvement for voice channels by combining the RF signals received by each directional sector antenna of the antenna system.